I. Field of the Invention
This invention relates generally to an electronic nerve stimulating device and more particularly to the improved design of a peripheral nerve stimulator useful in monitoring the nature of neurological blocks present in a patient following the administration of muscle relaxant drugs to the patient.
II. Description of the Prior Art
As is disclosed in the Ide et al U.S. Pat. No. 3,364,929, prior to surgery it is a common practice to administer muscle relaxant drugs such as succinylcholine and dimethyl tubocurare to the patient. Such drugs are found to produce depolarization or non-depolarization types of neuromuscular blocks in patients. Oftentimes, following surgery, it is desired that an antagonist drug be administered to counteract the effects of the muscle relaxant drug previously administered. It is also known that the antagonist drug should be introduced into the patient only when a non-depolarizing block exists in that the introduction of an antagonist drug when a depolarizing block is present is found to potentiate the depolarizing block rather than counteracting the muscle relaxant drug.
As is further set out in the Ide et al U.S. Pat. No. 3,364,929 and the Ide U.S. Pat. No. 3,565,080, the type of block extant within the patient, i.e., depolarizing or non-depolarizing, can be determined by applying electrical stimulation to a peripheral member of the body of the patient and noting the effect of that stimulation on the patient's digits. For example, if electrical pulses of a relatively low repetition rate are applied to the ulnar nerve, a characteristic twitch may be observed in the fingers of the patient. If a stimulation signal of a higher repetition rate is applied to the ulnar nerve, the fingers exhibit a constant contraction called tetanus. If, following the application of the relatively high frequency stimulating pulses, the relatively low frequency pulses are again applied, two different types of twitch reaction take place depending upon whether the patient is exhibiting a non-depolarizing block or a depolarizing block.
Objective evaluation of muscle relaxation during surgery is possible only by indirect stimulation of a skeletal muscle by means of a peripheral nerve stimulator. Most frequently, the ulner nerve is electrically stimulated and the resulting contraction of the hand muscles, i.e., twitching of the fingers is observed. This direct observation of the finger twitches induced by a nerve stimulator provides valuable information (a) on the actual magnitude of neuromuscular blockade (b) on the development of a dual block following suxanethonium, (c) on the need for more muscle relaxant, (d) on the reversal of muscle paralysis by a cholinesterase inhibiting agent, and finally (e) on the differential disagnosis of prolonged postoperative aponea. By employing a nerve stimulator, the neuromuscular blocking agents can be titrated according to the surgical requirements and overdosage can be avoided. By making use of the peripheral nerve stimulator of the present invention, the clinical judgment of muscle relaxation by the anaesthesiologist can be accompanied by a more quantitative and objective evaluation.
In a paper entitled "train-of-Four Nerve Stimulation in the Management of Prolonged Neuromuscular Blockade following Succinylcholine" which was published in the January 1975 issue of Anesthesiology, the authors, J. Savarese et al, reported on the value of the response evoked when nerve stimulating pulses were applied in groups of four and at a frequency of approximately 2 Hz. Similarly, in the October 1972 issue of Anesthesiology, there appeared a paper by B. Waud et al entitled "The Relation Between the Response to "Train-of-Four" Stimulation and Receptor Occlusion During Competitive Neuromuscular Block". Thus, in the field of anesthesiology, a peripheral nerve stimulator which can be selectively made to emit relatively high frequency tetanizing pulses, relatively low frequency twitch pulses and Train-of-Four pulse patterns is a useful and valuable tool.